Antennas



Nov. 13, 1956 F. J. LUNDBURG ANTENNAS Filed June 2, 1951 52j Iraq TATIGN IDENTIFICATION T KANSMITTER INVENTOR FRANK J. LUNQBURG BY) ATTO RNEY

United States Patent ANTENNAS Frank J. Lundburg, East Orange, N. J., assignor to International Telephone and Telegraph Corporation, a corporation of Maryland Application June 2, 1951, Serial No. 229,629

16 Claims. (Cl. 343-757) This invention relates to antennas and more particularly to a loop type of antenna and certain of its applications in antenna assemblies and arrays.

In the copending application of F. I. Lundburg and F. X. Bucher, Serial No. 138,138, filed March 12, 1950, now Patent No. 2,640,930, an antenna assembly is disclosed for use as an omnidirectional radio beacon. The antenna assembly of the radio beacon comprises a cage structure, containing a dipole antenna and including in the structure of the cage an additional antenna arrangement disposed symmetrically with respect to the dipole. The dipole antenna is of small length relative to a halfwave length of the center operating frequency and, there- (fore, presents a capacitive reactance. The cage structure disposed thereabout provides in effect a resonator for the dipole which reradiates for the dipole. The cage is cylindrical and includes conductive end pieces which are interconnected by circularly disposed vertical rods. The additional antenna arrangement comprises a plurality of .radiating elements spaced symmetrically about the cage in supported relation between certain of the rod thereof. To compensate for the capacitve reactance of the short dipole there is'provided within the cage a second cylindrical cage structure also formed of a plurality of spaced rods disposed about the dipole. The upper portions of the rods of the smaller cage support a conductive plate so positioned vertically with respect to the dipole as to provide an inner resonator cage structure of inductive reactance of a desired value sufficien-t to compensate for the capacitive reactance of the short dipole.

One of the objects of the present invention is to provide an improved omnidirectional radio beacon having an antenna which not only provides circular radiation but also assists in providing the desired resonance loading for the dipole.

Another object of the invention is to provide an improved form of loop antenna capable of substantially perfect circular radi tion and a further object is to provide such a loop antenna applicable for use in various antenna arrays.

Briefly, the loop antenna [feature of this invention comprises a slotted circular disc wherein the slots divide the peripheral portion of the disc into equal radiating sectors. The slots may be of any "selected configuration so long as the slots are of the same configuration and of corresponding loca-tion'with respect to the periphery of the disc. Straight slots radially-disposed in the disc are preferred. The disc antenna is fed in phase at the slots. The electrical length of the slots is determined by means of shorting bars whereby substantially perfect circular radiation is obtainable.

In the. omnidirectional beacon feature of the invention, the loop antenna serves two important functions. One function is the circular, horizontally polarized radiation obtainable by means of the loop antenna and the other function is its use in the resonant loading required for Patented Nov. 13, 1956 2 compensation of the capacitive reactance of the small size dipole. Y

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a view in side elevation of an omnidirectional radio beacon with parts broken away illustrating one embodiment of the invention;

Fig. 1A is a fragmentary sectionalview showing a detail of the beacon;

Fig. 2 is a cross-sectional view taken along line 22 of Fig. 1;

Fig. 3 is a diagrammatica-l illustration in side elevation of a stacked array of loop antennas; and

Fig. 4 is a diagrammatical illustration of a localizer array showing another application of the loop antenna of the invention.

Referring to the drawing, (Fig. 1 shows an omnidirectional radio beacon having as an element of the combination thereof, the double function loop antenna feature of this invention. The beacon includes a counterpoise 1 above which is disposed a small rotatable dipole 2. The dipole is driven by a motor 3 which is supported on a dielectric platform 4. The dipole is rotated at 1800 R. P. M. to produce a rotating figure-of-eight radiation.

Above the dipole i a loop antenna 5 adapted to produce a circular radiation. Surrounding the antennas 2 and 5 is a first or outer cylindrical cage 6 comprised of bottom and top conductive pieces such as"indicated by the top piece 7. The bottom piece may comprise'the center portion of the counter-poise as a separate part. These two end pieces as well as the counter-poise :1 may be solid or foraminous. The cylindrical wall of the cage comprises a plurality of spaced conductive rods 8/ This outer cage functions as a resonator excited by the dipole 2 and loop antenna 5. The spacing of the end pieces at 1 and 7 is preferably selected slightly greater than a half-wavelength of the center operating frequency. The rods '8 are spaced sufliciently close together to provide an effective vertical polarization screen or filter so that only the horizontal polarized energy passes. 'Elach two adjacent rods 8 form effectively the boundarie of a short "section of waveguide which acts as the ultimate radiators. As it is longer than ahalf-wavelength it will freely pass radiated energy in theTEm mode.

The structure so far described will provide a rotated radiation field pattern in the shape of a cardioid which will be generally horizontally polarized. However, due to the resonator action of the cage a certain amount of vertical polarization may be radiated from the cage structure due to radiation about the upper and lower ends ofthe cage. This vertical polarization is substan tially entirely overcome by extending the cage-structure upwafdly above the cage as indicated byv the three sections 9, 10 and 11. In one model, the upper structure 9, 10 and -1i, was made about 7 feet above the cage 8. All except a small percent of vertical polarization was eliminated by the 7 foot extension. In another model, the upper extended structure was made to extend 12 feet above the cage '7 and this 12 foot extension overcame, for all practical purposes, all undesired vertical polarization.

Since the dipole antenna 2 is short with respect to the wavelength it would normally tend to have a low radiation resistance and an undesirable high capacitive react-v The resonator formed by the cage 8 does not ance. fully overcome this deficiency of short dipole. In order to load the dipole antenna properly to achieve a desired compensation, the loop antenna 5 is mounted and formed 3 as a part of a second or inner cage 12 composed of a plurality of circularly disposed vertical rods 13. The loop antenna is in the form of a slotted disc 14 which may .be adjusted vertically onthe rods 13 to provide the;

desired resonance loading for the dipole 2 andthereby compensate for the capacitive reactance thereof and thus, obtain radiation efliciency. When the disc 14 is properly adjusted, the spacing thereof with respect to the lower endof the cage 12 will be less than a halfwavelength at the mid-operating frequency so that the resonator action of the cage 12 will be inductive. After vertical adjustment of the antenna disc 14 there may remain a certain mismatch between the antenna assembly and the transmission linet15 connected to the dipole 2. This mismatch is at least partially compensated for by providing open end coaxial stubs 16 at the central feeding point for the loop antenna, the loop antenna being fed by a coaxial line 17 through one of the bars 18 of the outer cage. Any number of stubs 16 may be employed as desired. areused to obtain partial matching. The remaining matching required is had at a convenient location by including in line 17 an adjustable impedance matching device 16a which may be a form of adjustable stub.

For a more complete understanding of the structural detail and operation of the loop antenna reference should be made to Fig. 2. The conductive disc 14 is circular and of copper, brass, aluminum or other high conductive material. The disc is provided with four radial slots, 19, 20, 21 and 22 disposed at right angles thereby subdividing the disc into four equal sectors. Any number of slots, however, could be employed, the number being determined to insure a constant current distributionfor a given diameter disc. These slots need not be straight and radial but may be of other configurations so long as all of the slots contained in the antenna disc are of substantially the same configuration. The loop antenna is fed by applying a potential across the four slots in the peripheral portion of the disc. The coaxial line 17, Fig. l, is brought to the center of the disc to a junction box 23, whereby-the feed is divided between the four slots, each slot being fed by a coaxial line 24 extending from the box 23 to the outer end of the slot where the inner conductor 25 is exposed crosswise of the slot. length of coaxial line 26 extends beyond the exposed center conductor 25 and constitutes an open end coaxial stub for matching purposes. It will thus be apparent that each of the four slots is fed at the outer ends thereof and that the feeding thereof is cophasal. The electrical length of the slots 19, 20, 21 and 22 is determined by shorting bars, such as indicated at 27, the shorting bars being adjustable so that a substantially perfect circular radiation can be obtained.

The radiation function of the loop will be apparent by of concentric loops all derive their excitation from the -l' radial slots 19, 20, 21 and 22. The coaxial lines 24 and 26 for each slot may be varied in length to satisfy the impedance at the junction box 23. The coaxial line 26 is usually an open end line to provide capacitive impedance to nullify the inductive reactance of the. loop.

Besides this function these lines 24 and 26 also serve to provide an effective short for the feed line at the junction box 23 with respect to the excitation from the dipole 2. This will be understood by reference to. Fig. 1 wherein the loop antenna 5 constitutes the upper end piece for the inner resonator-cage 12. The axis of the rotating dipole 2 is coaxial with respect to the center of the loop antenna 5. In this relationship it is desirable that the loop antenna appear as a solid conducting disc to the dipole. The dipole induces out-of phase voltages across diametrically opposite In the present embodiment four A second slots in the disc 14. These voltages cause currents to travel down the coaxial lines 24 and due to the phase of excitation of the dipole a virtual short is produced at the junction box 23. By selecting proper lengths of the coaxial lines 24 and 26 this short at the junction box 23 is reflected across the slot at the exposed conductor 25. Thus, insofar as the dipole is concerned, the loop antenna appears as a solid conducting disc with no mutual interaction.

In the operation of the beacon, two standby transmitter equipments 28 and 29 are normally provided. Each such equipment comprises a transmitter unit 30 for impressing on the carrier portion of the antenna radiation, station identification, voice modulation and a 30 C. P. S. reference signal which is a function of the angular position of the rotating dipole. The output of the transmitter 30 is applied to a modulation eliminator 31 which has two outputs, one to a relay 32- and thence through transmission line to the dipole 2, and the other through aphaser 33 to a second relay 34 for application through the adjustable matching device 160 and transmission line 17 to the loop antenna 5. The rotating antenna 2 is driven by motor 3 from a power source 36. The rotation of the dipole 2 produces the ref erence signal by means of a tone-wheel 37 driven from the shaft of motor 3. This reference signal, which is applied to the transmitter 31). through connection 38, is frequency modulated from 9.43 to 10.44 kc. at a rate of C. P. S. This reference signal modulates the VHF signal that feeds the loop antenna. All modulation is removed from the portions of the V.H.F. signal that feeds the dipole.

From the foregoing description of the omnidirectional beacon it will be clear that an omnidirectional radiation pattern is obtained by means of the rotating dipole 2, the loop antenna 5 and the associated resonant cage structures 6 and 12. The loop antenna not only supplies the circular radiation whereby a rotating cardioid radiation pattern is obtained but also it serves in conjunction with the resonant cage 12 to compensate for the capacitive reactance of the dipole due to the small size thereof with respect to the operating frequency.

While the loop antenna is shown in the embodiment of Fig. l as having a double function therein, the loop antenna may be used in many other antenna systems, two of which are illustrated, by way of example, in Figs. 3 and 4. In Fig. 3, the loop antennas are stacked in serial order one-half wavelength apart on a metal pipe support 39. While only three loop antennas 40, 41 and 42 are shown, it will be clear that many more may be included as may be desired. In such an array alternate loop antennas are inverted to provide for a desired 180 phase reversal in the feeding relationship thereof. This phase reversal is obtained by positioning the antenna upside down, such as antenna 41. The array may be provided with a counter-poise 43 if desired. The array of this type utilizing the loop antennas provide horizontally polarized circular radiation and is particularly adaptable for FM broadcast use.

In Fig. 4, two loop antennas 44 and 45 are shown supported in laterally spaced relation above a counter-poise 46. This arrangement is particularly useful as a localizer. By disposing one of the loops upside down as indicated by the antenna 45 the desired phase opposition is obtained withoutthe usual transposition. The feed for the two antennas 44 and 45'is equally balanced from the common feed point 47. Should it be desired to have the two antennas operate in phase, the two may be positioned alike.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention, as set forth in the objects thereof and in the accompanying claims.

I claim:

1. An antenna system comprising a dipole antenna adapted to be rotated to provide a radiation of horizontal polarization in the form of a figure-of-eight pattern, a loop antenna of circular disc configuration, having slots therein and adapted to provide a radiation of horizontal polarization in the form of a circular pattern whereby the two radiations combine to provide a rotating cardioid pattern, said dipole antenna being small compared to onehalf of a wavelength of the operating frequency thereby presenting high capacitive reactance, means to feed said dipole antenna, means to feed said loop antenna including means to effectively short circuit said slots with respect to currents induced in said disc by said rotating dipole, and means including said loop antenna to provide a resonant loading to compensate for the capacitance reactance of said dipole antenna.

2. An antenna system according to claim 1, wherein said means to provide a resonant loading comprises a resonant cage structure disposed about said dipole antenna, said resonant cage structure being substantially cylindrical and having said loop antenna of circular disc configuration disposed as one of the end portions thereof.

3. An antenna system according to claim 2, further including a second resonant cage structure disposed about said first mentioned cage structure, said second cage structure comprising a plurality of rods of conductive material disposed in spaced relation circumferentially of said first mentioned cage to form effectively a polarization filter, and an extension of one of said cage structures at one end thereof for attenuating any component radiation energy polarized perpendicular to the plane of polarization of said dipole and loop antennas.

4. An antenna system comprising a dipole antenna adapted to be rotated to provide a radiation of horizontal polarization in the form of a figure-of-eight pattern, a loop antenna of cincular disc configuration adapted to provide a radiation of horizontal polarization in the form of a circular pattern whereby the two radiations combine to provide a rotating cardioid pattern, said dipole antenna being small compared to one-half of a wavelength of the operating frequency thereby presenting high capacitive reactance, means to feed said dipole antenna, means to feed said loop antenna, means including said loop antenna to provide a resonant loading to compensate for the capacitance reactance of said dipole antenna, said loop antenna of circular disc configuration having a plurality of slots dividing the peripheral portion thereof into arcuate sectors, and means to feed said disc cophasally at said siots adjacent the periphery of said disc whereby arcuate curr nts are caused to flow in the said sectors.

5. An antenna system according to claim 4, wherein said disc is provided with shorting elements spanning said slots, said shorting elements being adjustable lengthwise of said slots relative the location of the feeding means for said siots to determine the electrical lengths of the slots whereby a substantially perfect circular radiation pattern may be obtained.

6. An antenna system according to claim 4, wherein said loop antenna is provided with a balanced feed connection for said slots and a plurality of capacitive stubs coupled to said feed connection.

7. An antenna system comprising a circular disc of conductive material having a plurality of slots dividing the peripheral portion thereof into a plurality of arcuate sectors, and means to feed said disc cophasally at the ends of said slots adjacent the periphery of said disc whereby arcuate currents are caused to flow in the conductive sections between adjacent slots for radial outward radiation.

8. An antenna system according to claim 7, wherein said disc is provided with shorting elements spanning said slots, said shorting elements being adjustable lengthwise of said slots relative the periphery of said disc to determine the electrical lengths of the slots whereby a substantially perfect circular radiation pattern may be obtained.

9. An antenna system comprising a plurality of circulat slotted discs arranged in an array, each of said discs having the slots'therein disposed to divide the peripheral portion of the disc into a plurality of arcuate sectors, means to feed each disc cophasally at said slots whereby arcuate currents are caused to flow in the conductive sectors between adjacent slots for radial outward radiation, and means to feed the individual feeding means of said discs in phase, alternate discs of said array being inverted to provide for C. phase reversal.

10. An antenna system according to claim 9, wherein the array of discs is in the form of a stacked series spaced substantially one-half wavelength of the operating frequency.

11. An antenna system according to claim 9, wherein the discs of the array are disposed in spaced lateral relation.

12. An antenna system according to claim 7, further including a resonant cage structure of substantially cylindrical shape and means disposing said circular disc as an end portion for said cage.

13. An antenna system according to claim 12, further including an antenna contained within said cylinder in spaced relation to said circular disc.

14. An antenna. system comprising a dipole antenna adapted to be rotated to provide a radiation of horizontal polarization in the form of a figure-of-cight pattern, a loop antenna of circular disc configuration adapted to rovide a radiation of horizontal polarization in the form of a circular pattern whereby the two radiations combine to provide a rotating cardioid pattern, said dipole antenna being small compared to one-half of a wave-length of the operating frequency thereby presenting high capacitive reactance, means including said loop disc antenna to provide a resonant loading to compensate for the capacitance reactance of said dipole antenna, said disc loop antenna having a plurality of slots dividing the peripheral portion thereof into sections, a coaxial feed line extending to the outer ends of each of said slots Where the inner conductor is exposed crosswise of the slot to thereby feed said slots cophasally and cause current to flow in said sections, said coaxial line being continued beyond said exposed part as an open end stub, whereby a resonant inductive capacitance relationship is obtained at the feed end of said slots with regard to excitation from said dipole whereby said slotted disc antenna appears to the dipole antenna as a solid disc conductor.

15. An antenna system comprising circular disc of conductive material having a plurality of siots divid ng the peripheral portion thereof, and means to feed said disc cophasally at said slots whereby arcuate currents are caused to flow in the conductive sections between adjacent slots for radial outward radiation, said means to feed said disc including a coaxial line extending to the outer ends of said slots where the inner conductor is exposed crosswise of said slots, said coaxial line being continued beyond said exposed part as an impedance matching stub.

16. An antenna system according to claim 15, w erein the coaxial feed lines for said slots are joined to a common feed connection equi-dist-ant from the points Where the inner conductor is exposed for feeding purposes.

References Cited in the file of this patent UNITED STATES PATENTS 2,267,613 Lindenblad Dec. 23, 1941 2,471,021 Bradley May 24, 1949 2,532,919 Johnson Dec. 5, 1950 2,532,920 Johnson Dec. 5. 1950 2,557,951 De Rosa June 26, 1951 2,600,179 Alford June 10, 1952 2,617,033 Posthumus Nov. 4, 1952 2,622,199 Ramsey et al. Dec. 16, 1952 2,640,930 Lundburg June 2, 1953 

